Inline `_check_event_auth` for outliers (#10926)

* Inline `_check_event_auth` for outliers

When we are persisting an outlier, most of `_check_event_auth` is redundant:

 * `_update_auth_events_and_context_for_auth` does nothing, because the
   `input_auth_events` are (now) exactly the event's auth_events,
   which means that `missing_auth` is empty.

 * we don't care about soft-fail, kicking guest users or `send_on_behalf_of`
   for outliers

... so the only thing that matters is the auth itself, so let's just do that.

* `_auth_and_persist_fetched_events_inner`: de-async `prep`

`prep` no longer calls any `async` methods, so let's make it synchronous.

* Simplify `_check_event_auth`

We no longer need to support outliers here, which makes things rather simpler.

* changelog

* lint

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